1. Field of the Invention
The invention relates to substrates used in out-of-ground cultivation techniques. More precisely, the invention relates to substrates used in cultivation techniques that employ successive stages of growth of the seedlings, with each stage leading to the use of a substrate offering a volume greater than that of the preceding one to make possible a satisfactory development of the rooting system of the seedling.
The invention also relates to mineral substrates on which the seedlings are developed calling for only a limited volume on the order of magnitude of 1 liter. More precisely, the invention relates to substrates of such mineral fibers as those consisting of glass wool or mineral wool.
2. Discussion of the Background
In intensive methods of out-of-ground cultivation, good management of materials and of available space leads to a sequence of stages corresponding to the use of specific substrates. The most widely used method at present in out-of-ground cultivation in the greenhouse comprises at least two successive stages. The first stage corresponds to the initial growth of the seedlings. As the seedlings are not very large and exhibit limited development of the rooting system, a substrate of restricted volume is preferred at this stage. This makes it possible to increase the number of seedlings cultivated in a limited space. This also makes it possible to minimize the volume of nutritive solution necessary to maintain the substrate under suitable conditions for the development of seedlings.
In the second stage, the seedlings which have grown tall on these limited volume substrates are placed, along with the first substrate, on a second substrate offering a larger volume, and ordinarily spaced from one another.
In the methods of intensive cultivation which represent the most significant prospects for this type of product substrates are used having a cubic shape and will hereinafter be referred to as "cubes". The cubes are placed against one another, in a first step, on a wide surface. In this type of arrangement, it is necessary to provide both satisfactory irrigation and suitable aeration to assure the most complete and the quickest growth of the seedlings. The irrigation can be performed either from the upper part of the cubes, or from their base, i.e. the part resting on the ground. In practice, the two irrigation methods can be combined.
Often, the cubes are placed in a unit which forms a sort of tank capable of receiving enough irrigation solution to partially submerge the cubes. The solution rises to a sufficient level relative to the height of the cubes so that the cubes are completely saturated. For cubes of ten centimeters or so in height, the temporary level of the irrigation solution reaches, for example, a third of the height of the cube. The solution then rises in the substrate by capillary action. With this irrigation performed, the irrigation solution is drained and recovered for a later operation. This type of irrigation operation is repeated at a frequency depending both on the cultivation being considered and the ambient conditions, season, temperature, evaporation, etc.
After a certain development of the seedlings, it is normal to separate the cubes from one another to offer more space and light. At this stage also the irrigation is advantageously performed as indicated above, a method that is designated by the term "subirrigation."
In these two stages of cultivation in cubes, it was pointed out that it was necessary to assure good aeration of the roots between the successive irrigations. This aeration is a necessary condition to good development of the roots. The traditional cubes used exhibit a level base resting completely on the ground or, more exactly, on the bottom of a tank or on a covering, most often an impermeable film covering the ground and separating the cubes from the ground. In all these applications, a difficulty arises. The lower face of the cubes tends, in contact with the support on which it rests, to maintain a liquid film after the irrigation has stopped. In other words, when the irrigation solution is evacuated, whether the solution has been introduced by subirrigation or by irrigation through the upper face of the cubes, the contact between the cube and its support maintains, by what can be named a surface effect, a continuous liquid film which is opposed to the normal drainage of the cube under the effect of gravity. Under these conditions, water retention is extended substantially, resulting in the establishment of an unfavorable air/water ratio.
To reestablish a quick drainage of the solution after each irrigation operation, a technique developed by the users has been to place under the cubes a porous material or slatted floors which eliminate this surface effect by breaking the contact between the cubes and their support. These means, however, have the drawback of complicating the maintenance of the cultivation areas. In particular, they must be sterilized between each new cultivation.
Another solution used consisted of making grooves on the lower face of the cubes. These grooves, which are on the order of 1 to 2 centimeters in depth and width, divide the film in a way which improves the drainage and, as a result, the aeration of the cubes. These grooves make possible the reduction of the surface of contact by about one third and the return to a suitable air/water balance in an appreciably shortened period after each irrigation. A surface consistently in contact with the support also remains, however, at the base of the cube, and, although very small, the underlying liquid film remains present at these points of contact.